In states where vapor recovery at gasoline retail stations is required, booted filling nozzles are used. Vapors emitted during filling are redirected to the underground storage tanks via a boot around the filling pipe. However, the clumsiness of the nozzle and the imperfect fit of the boot on the tank pipe cause many gasoline stations to install bootless, vacuum-assisted recovery systems. The bootless system pulls in air via the nozzle and redirects the hydrocarbon vapor/air mixture to the underground tank.
The flow rate of gasoline going to the car and the flow rate of the vapor/air mixture returning to the storage tank must be exactly balanced to maintain constant ambient pressure inside the tank. If the pressure in the storage tank increases, gasoline vapors are emitted from the vent pipe of the tank. The composition of the vented vapors varies depending on the composition of the fuel itself and the environmental conditions. Most gasoline fuels consist of a mixture of straight- and branched-chain hydrocarbons, alcohols, ethers or other oxygen-containing compounds, and other minor components. The hydrocarbons range from light volatiles, such as C.sub.7 or below, to C.sub.12 or heavier, relatively involatile materials. Thus the vapor emissions also vary, but might typically consist of a mixture of C.sub.3 -C.sub.7 hydrocarbons, with other minor components. The flow rates of such vent emissions are generally small, less than 10 scfm and typically in the range 0.5-2.0 scfm.
Various technologies for controlling or destroying organic vapors in waste streams exist.
Several of these could, in principle, be used to control vent emissions from the fuel storage tanks at a gas station. Carbon adsorption is effective, but expensive due to high costs for carbon regeneration or replacement. Incineration creates a safety hazard, and the sight of an open flame at a gasoline retail station might cause concern among consumers. Catalytic oxidation requires a gas stream that is constant in volume and concentration. Since gasoline vapor emissions at retail stations fluctuate throughout the day, extensive controls are required, making this approach unattractive. Condensation is generally too costly, due to the low temperatures required for adequate gasoline vapor recovery.
It is known to apply membrane separation systems to the separation of organic vapors from other gases, and even specifically to the recovery of vent vapors. For example, U.S. Pat. No. 5,044,166 describes a membrane separation system for recovery of chlorofluorocarbon or other emissions from refrigeration purge vents. German Patent DE 42 25 170 A1, to Roland Pelzer, describes a system that maintains sub-atmospheric pressure in the underground fuel storage tank, while using a membrane system to recover air/fuel vapor emissions.
Even though membranes are available that have good intrinsic separation properties for hydrocarbon vapors over air, relatively poor separation performance can be achieved in practice. One factor that makes the use of membrane systems unattractive is the small flow rates of the streams to be treated. In a typical small spiral-wound module, containing about 1 m.sup.2 of membrane in a single leaf, a vent gas flow rate of, for example, 1 scfm translates to a bulk gas velocity of only 0.7 m/s or less within the feed channels of the module.
As with any fluid flowing across a surface, the velocity profile of the gas in the feed channels is not constant across the thickness of the channel, because of friction at the gas/membrane interface. The gas velocity decreases as the distance from the membrane surface decreases and a stagnant boundary layer is present near the membrane surface. The gas mixture concentration is uniform outside the stagnant boundary layer, because the flow is turbulent. However, the flow in the boundary layer is laminar, producing a concentration profile across this layer as the faster permeating components are removed preferentially through the membrane.
The effect of concentration polarization is that components that are enriched in the permeate are depleted in the boundary layer, and components that are depleted in the permeate are enriched in the boundary layer. For further separation of the faster-permeating components to occur, these must cross both the boundary layer and the membrane. Thus, the boundary layer acts as an additional resistance, in series with the membrane, to transport from the bulk feed to the permeate side of the membrane.
If other considerations make it possible, removal of the faster-permeating components from the feed stream can be improved by increasing the feed flow rate through the module, because this promotes turbulence, reduces the thickness of the boundary layer, and thereby reduces the boundary layer resistance. If a large membrane area is required to perform a separation, then concentration polarization problems can be addressed by dividing the membrane area between multiple small modules in series, rather than using one large module. This maintains a higher flow rate and a more turbulent flow.
In vent-stream applications of membrane technology, however, this is often not possible. If the flow rate of the vent gas is just a few scfm, then the membrane area required to treat the stream may be just a few square meters, such as 1 m.sup.2, 2 m.sup.2 or 5 m.sup.2, and dividing this small area between multiple modules may be difficult and costly.
Thus, despite the availability of diverse control technologies, there remains a need for better methods of controlling emissions from fuel transfer operations, in particular, emissions from automotive fuel dispensing at gasoline stations.
Another factor to be taken into account in addressing this problem is that the current bootless vapor recovery system could be improved if more air could be drawn in at the nozzle, and the air then selectively removed from the tank without loss of the fuel vapor. The hydrocarbon vapors could be recovered and returned to the tank, and the problem of fugitive emissions would be eliminated.
Of course, emissions control problems of this type are not limited to retail automobile fuelling. The same or similar considerations apply to other fuel transfer operations, such as transfer from underground tanks at tank farms into tanker trucks; from tanker trucks into underground storage tanks at gasoline retail stations and other fueling sites; from storage tanks into small tank trucks used for off-site refueling; and dispensing of fuels into vehicles other than automobiles, such as, trucks, construction vehicles, aircraft, boats, and ships. Likewise, in a more general sense, the same types of problems are confronted by owners and operators of all facilities that use tanks containing potentially volatile liquids, including fuels, solvents, reagents, and other organic and inorganic materials.
Co-owned and copending applications 08/535,983 and 08/536,633, which are incorporated herein by reference in their entirety, discuss spiral-wound modules that use baffles in the feed and/or permeate channel to produce counter-current feed/permeate flow patterns. The applications cite the following references as representative examples: U.S. Pat. Nos. 5,154,832; 5,096,584; 5,034,126; 4,814,079; 4,765,893; and 4,033,878.